Re: [PATCH RFC 2/3] locking/rwsem: clear reader-owner on unlock to reduce false positives

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 24 Jun 2025 13:02:31 +0800
Lance Yang <lance.yang@linux.dev> wrote:

> 
> 
> On 2025/6/24 11:53, Masami Hiramatsu (Google) wrote:
> > On Tue, 24 Jun 2025 09:44:55 +0800
> > Lance Yang <lance.yang@linux.dev> wrote:
> > 
> >>
> >>
> >> On 2025/6/24 08:26, Masami Hiramatsu (Google) wrote:
> >>> On Thu, 12 Jun 2025 12:19:25 +0800
> >>> Lance Yang <ioworker0@gmail.com> wrote:
> >>>
> >>>> From: Lance Yang <lance.yang@linux.dev>
> >>>>
> >>>> When CONFIG_DETECT_HUNG_TASK_BLOCKER is enabled, a stale owner pointer in a
> >>>> reader-owned rwsem can lead to false positives in blocker tracking.
> >>>>
> >>>> To mitigate this, let’s try to clear the owner field on unlock, as a NULL
> >>>> owner is better than a stale one for diagnostics.
> >>>
> >>> Can we merge this to [PATCH 1/3]? It seems that you removed #ifdef and
> >>> remove it. This means in anyway we need the feature enabled by DEBUG_RWSEMS.
> >>
> >> Thanks for the feedback! I see your point about the dependency ;)
> >>
> >> Personlly, I'd perfer to keep them separate. The reasoning is that
> >> they addreess two distinct things, and I think splitting them makes
> >> this series clearer and easier to review ;)
> >>
> >> Patch #1 focuses on "ownership tracking": Its only job is to make
> >> the existing owner-related helpers (rwsem_owner(), is_rwsem_reader_owned())
> >> globally available when blocker tracking is enabled.
> >>
> >> Patch #2, on the other hand, is about "reader-owner cleanup": It
> >> introduces a functional change to the unlock path, trying to clear
> >> the owner field for reader-owned rwsems.
> > 
> > But without clearing the owner, the owner information can be
> > broken, right? Since CONFIG_DEBUG_RWSEMS is working as it is,
> 
> You're right, the owner info would be broken without the cleanup logic
> in patch #2. But ...
> 
> > I think those cannot be decoupled. For example, comparing the
> > result of both DETECT_HUNG_TASK_BLOCKER and DEBUG_RWSEMS are
> > enabled and only DETECT_HUNG_TASK_BLOCKER is enabled, the
> > result is different.
> 
> The actual blocker tracking for rwsems is only turned on in patch #3.
> So, there's no case where the feature is active without the cleanup
> logic already being in place.
> 
> > 
> >>
> >> Does this reasoning make sense to you?
> > 
> > Sorry, no. I think "reader-owner cleanup" is a part of "ownership
> > tracking" as DEBUG_RWSEMS does (and that keeps consistency of
> > the ownership tracking behavior same as DEBUG_RWSEM).
> 
> I thought this step-by-step approach was a bit cleaner, since there are
> currently only two users for these owner helpers (DEBUG_RWSEMS and
> DETECT_HUNG_TASK_BLOCKER).

I think the step-by-step approach fits better if the feature is evolving
(a working feature is already there.) I don't like the intermediate
state which does not work correctly, because if we have a unit test(
like kUnit) it should fail. If you can say "this finds the rwsem
owner as same as what the CONFIG_DEBUG_RWSEM is doing", it is simpler
to explain what you are doing, and easy to understand.

> 
> Anyway, if you still feel strongly that they should be merged, I'm happy
> to rework the series as you suggested ;p

Thanks,

> 
> Thanks,
> Lance
> 
> > 
> > Thank you,
> > 
> >>
> >> Thanks,
> >> Lance
> >>
> >>>
> >>> Thanks,
> >>>
> >>>>
> >>>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> >>>> ---
> >>>>    kernel/locking/rwsem.c | 10 ++++------
> >>>>    1 file changed, 4 insertions(+), 6 deletions(-)
> >>>>
> >>>> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> >>>> index 6cb29442d4fc..a310eb9896de 100644
> >>>> --- a/kernel/locking/rwsem.c
> >>>> +++ b/kernel/locking/rwsem.c
> >>>> @@ -205,14 +205,12 @@ bool is_rwsem_reader_owned(struct rw_semaphore *sem)
> >>>>    		return false;
> >>>>    	return rwsem_test_oflags(sem, RWSEM_READER_OWNED);
> >>>>    }
> >>>> -#endif
> >>>>    
> >>>> -#ifdef CONFIG_DEBUG_RWSEMS
> >>>>    /*
> >>>> - * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
> >>>> - * is a task pointer in owner of a reader-owned rwsem, it will be the
> >>>> - * real owner or one of the real owners. The only exception is when the
> >>>> - * unlock is done by up_read_non_owner().
> >>>> + * With CONFIG_DEBUG_RWSEMS or CONFIG_DETECT_HUNG_TASK_BLOCKER configured,
> >>>> + * it will make sure that the owner field of a reader-owned rwsem either
> >>>> + * points to a real reader-owner(s) or gets cleared. The only exception is
> >>>> + * when the unlock is done by up_read_non_owner().
> >>>>     */
> >>>>    static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
> >>>>    {
> >>>> -- 
> >>>> 2.49.0
> >>>>
> >>>
> >>>
> >>
> > 
> > 
> 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>